There are already known electric welding apparatus that comprise controlled rectifiers (thyristors). This and similar types of welding apparatus, however, will not do without voluminous and heavy-weight power transformers.
On the other hand, electronically controlled power supplies for small and medium power outputs, which are designed as socalled switched-mode power supplies (SMPS), are known primarily from laboratory technology and entertainment electronics. There, the power voltage at first is rectified, then "chopped" by a high switching frequency (mostly above 20 kHz), transformed to the desired voltage via a transformer, and finally rectified anew. By chosing a high switching frequency, the efficiency can be increased on the one hand and the volume and weight of the transformer can be considerably reduced on the other hand. There is a difference between primarily controlled and secondarily controlled switched mode power supplies, the control being effected either by changing the clock time at a constant frequency or by changing the frequency at a constant operation pulse duration. Mostly, quick power transistors with high operational voltages are used.
The field of application of switched mode power supplies so far has been restricted to television sets, LF power amplifiers, measuring devices, film and still projectors, accounting, calculating and typing machines, computers, voltage and current stabilizers for industry and laboratories, power supplies for fluorescent lamps, HF and LF generators, motor control, accumulator chargers, AC/AC-converters and low voltage power supplies with galvanic power separation. Depending on the output (1 to 3,000 W and more), the following types of converters are chosen with an increased output: single-ended boost regulators, single-ended buck regulators, push-pull half bridge, push-pull full bridge and push-pull parallel feed.
In particular with switched mode power supplies of higher outputs the problem arises that, due to power overvoltages or to line-side short-circuits, the admissible limits U.sub.max, J.sub.max of the transistors used can be exceeded, which leads to a destruction of the transistors. Therefore, complex safety cut-out and operation-sequence circuits are required, e.g. overvoltage, peak current, overload, short-circuit fuse protections, switch-on and restarting measures and the like, which are mostly unnecessary with switched-mode power supplies of smaller outputs, since there the transistors may be chosen to be overdimensioned from the beginning in order to offer a certain safety. Therefore there has been a prejudice to building switched-mode power supplies of higher outputs, which are necessary e.g. for electric welding apparatus (more than 10,000 W).